Swend
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It seems a perfectly plausible artifact considering the presence of inductors and capacitors at the output stage performing a tank circuit capable of sustaining an oscillation during the recharge of the capacitors.
By the way, it would be more clarifying if you added some resistive load to the output, so you would see how much that ringing 'noise' would remain in the face of the peak current in the capacitor recharge (ripple) in the secondary side of transformer.
The noise is at 40khz and your switching is at 40khz so it's switching noise correct?
First figure out of its 'real' by tying your scope probe to ground and see if its scope noise or common mode pickup.
If not then try more/better capcitors, diode snubbers, and/or a post LC filter.
Yup the noise is at 40kHz intervals - so it is switching noise - most likely from the o/p diodes - put snubbers across all.
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yes - those diodes are 300nS recovery - so they carry reverse current when they turn off - they are really designed for lower frequencies than 40kHz, I see you have tried to mitigate this with a sine wave source - however they will need snubbers - or use faster & better 600V diodes - now with smaller snubbers ...
at a guess 330ohm and 220pF ( 1 or 2kV ) across each diode.
a small LC post filter will do a lot to remove noise too ... esp for a low C choke ...
Wait, let's be really clear. When you say "The noise is 'real' - grounding does nothing" does that mean you still saw the noise after grounding the probe tip?
I meant you should leave your probe ground connected and clip or touch the probe to its own ground lead. This should obviously show a perfect 0V. If you still see noise then its scope pickup and isn't actually on your output. The scope pickup may be worth trying to solve...but its a different problem.
LC filter, in the uF HV range I only had a 50uF/800V so I tried that in combination with several different coils just to see if any change could be registered e.g. 50uF/10mH I get Fc=7KHz, but no significant change could be registered.
this likely means that you are picking up only CM noise on your scope - taking noise measurements at low levels is an art, try looping your scope probe as many times as you can thru a high mu toroid - this will reduce CM pickup ...
for a post filter you need a low capacitance choke - say 100uH, and a low ESR/ESL cap ( i.e. no long leads ) to get attenuation at the noise freq, say 220nF, film/foil.
using caps only as a snubber is counter productive ...
i think OP has problem with "pickup" in his scope lead, as many others here have suggested.
You can use a coaxial probe (you have to make it yourself), which doesnt have the ground clip.
to remove noise pickup use probe with spring https://electronics.stackexchange.com/questions/221292/tip-barrel-test-of-oscilloscope
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